In the semiconductor industry there is a growing need for metallization processes, particularly copper metallization processes, that can be used to grow high purity metal films, such as copper films, onto silicon wafer surfaces. These copper films then form the basis for creating the microscopic high speed electrical interconnect pathways or `lines` that are buried inside the architecture of advanced microprocessors. A key technology for fabricating these metal films, such as copper films, is Chemical Vapor Deposition (CVD). In this approach, a metal (i.e., copper) containing chemical vapor is contacted with a heated substrate surface in such a way that a surface chemical reaction occurs to deposit a pure metal (i.e., copper) film. To ensure the purity of the metal film, the CVD precursor must itself be of high purity. For example, the leading copper precursors for the CVD of copper films are of the type copper.sup.(+1) (.beta.-diketonate)(A), where (A) is a neutral ligand usually of the unsaturated type. Within this class of compounds the leading candidate is copper.sup.(+1) (hexafluoro-2,4-pentanedionate) (trimethylvinylsilane), known commercially as CupraSelect.RTM. precursor available from the Schumacher unit of Air Products and Chemicals, Inc., Carlsbad, Calif.
This compound and its synthesis has been described in U.S. Pat. No. 5,144,049 by the synthesis route using CuCl rather than Cu.sub.2 O as the metal compound precursor.
The synthesis of other similar metal .beta.-diketonates is described in "Alkene and Carbon Monoxide Derivatives of Copper(I) and Silver(I) .beta.-Diketonates", by G. Doyle, et. al., Organometallics, Vol. 4, No. 5, 1985, pp. 830-835.
Doyle also obtained several patents on the use of copper .beta.-diketonates as adsorbents for various unsaturated materials, i.e., U.S. Pat. No. 4,279,874; U.S. Pat. No. 4,385,005; U.S. Pat. No. 4,425,281; U.S. Pat. No. 4,434,317; and U.S. Pat. No. 4,471,152.
Dehydration of this type of compound after synthesis using the Cu.sub.2 O synthesis route is described in U.S. Pat. No. 5,663,391 where anhydrous copper sulfate was added to the synthesis mixture to achieve dehydration of the desired product.
Typically, during the synthesis of this class of compounds, undesired side reactions occur to form undesired side products which must then be removed. Specifically, in the case where the .beta.-diketone is hexafluoro-2,4-pentanedione, an undesired side reaction product that can form is Cu.sup.+2 (hexafluoro-2,4-pentanedionate).sub.2. This dark blue colored solid copper complex dissolves into the desired copper.sup.(+1) (.beta.-diketonate)(A) product, and since the latter species are typically bright yellow, the resulting solution is green. High concentrations of the Cu.sup.+2 (hexafluoro-2,4-pentanedionate).sub.2 by-product lead to inefficiencies in subsequent purification processes resulting in lowered yields of the desired copper.sup.(+1) (hexafluoro-2,4-pentanedionate)(A) precursor.
The present invention describes how the synthesis of these metal-ligand complexes, such as copper.sup.(+1) (hexafluoro-2,4-pentanedionate)(A), can be carried out in such a way that the formation of the undesired higher valence metal-ligand complexes, such as Cu.sup.+2 (hexafluoro-2,4-pentanedionate).sub.2, is suppressed. The resulting lowered concentration of higher valence metal-ligand complexes (i.e., Cu.sup.+2 (hexafluoro-2,4-pentanedionate).sub.2) in the crude reaction product thus facilitates more efficient purification steps and consequently a higher yield of pure product.